The present invention relates generally to wireless communications systems and, in particular, to power control in wireless communications systems having multiple information rates.
Wireless communications systems use power control to improve system performance and increase system capacity. Power control involves tracking possible fading of communication channels and using that tracked fading to manage the power at which signals are being transmitted from base stations (in order to compensate for the fading). Conventional Code Division Multiple Access (CDMA) wireless communications systems based on the well-known IS-95 standard use error indicator bits to assist in controlling transmission power at the base station. Power control is implemented in the following manner.
When a call is set up in a CDMA wireless communications system, a base station and a mobile station communicate over a forward and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile station and the reverse link includes communication channels for transmitting signals from the mobile station to the base station. The base station transmits control information to the mobile station over a communication channel referred to herein as a forward control channel, and the mobile station transmits control information to the base station over a communication channel referred to herein as a reverse control channel. The base station transmits voice or data to the mobile station over a communication channel referred to herein as a forward traffic channel, and the mobile station transmits voice or data to the base station over a communication channel referred to herein as a reverse traffic channel. In either traffic, channel, voice or data is transmitted over 20 milli-seconds (ms) time intervals referred to herein as frames.
A set number of voice or data bits are transmitted within each frame, typically reported as the number of bits transmitted per second, referred to herein as a channel rate. The channel rate does not change and is typically dependent on the rate of the coder in the system, i.e. the rate of the speech or data coder. However, the amount of information within each frame of the signal does change, particularly for voice signals. Therefore the number of bits of information transmitted per second, referred to herein as the information rate, can change.
Four different information rates are possible in the traffic channel: full rate, xc2xd rate, xc2xc rate and xe2x85x9 rate. At the full rate the information rate and the channel rate are a equal. The information rate of the forward traffic channel is at the full rate when a large amount of information is being transmitted from the base station to the mobile. The information rate of the forward traffic channel is at the xe2x85x9 rate when a small amount of information is being transmitted from the base station to the mobile. The xc2xd and the xc2xc rate are transitional rates. For example, in a telephone conversation between a mobile station user and a second user communicating with the mobile station user, a voice signal from the second user is transmitted to the base station, which transmits it to the mobile station over the forward traffic channel. During a part of the conversation, the second user is talking. Therefore, the information rate of the forward traffic channel would be high because a large amount of information is being transmitted on the forward traffic channel. In this case, the information rate would be the full rate. During another part of the conversation, the second user is listening. Therefore, the information rate of the forward traffic channel would be low because a small amount of information is being transmitted on the forward traffic channel. In this case, the information rate would be equal to the xe2x85x9 rate.
When the information rate is xc2xd, xc2xc, or xe2x85x9, the channel rate is higher than the information rate, and the information is repeated several times per frame. For example, with the xc2xd rate information is repeated twice each frame; with the xc2xc rate the information is repeated four times per frame; and with the xe2x85x9 rate the information is repeated eight times per frame. Repeating the information several times per frame permits the information to be transmitted at a correspondingly lower power. The power is scaled by an information rate scaling factor, which is equal to the information rate. For a frame whose information rate is equal to the xe2x85x9 rate, the information rate scaling factor is xe2x85x9, and the power can be reduced to xe2x85x9 of the power of the frame at the full rate.
The bits in the frame are spread in time, referred to herein as interleaved. Interleaving typically spreads out important bits in time so that if there is a deep fade or noise burst the important bits are not corrupted by one deep fade or noise burst. This reduces the number of frames containing errors, referred herein as a frame error rate.
When system conditions are equal, frames that have an information rate lower than the channel rate have a lower frame error rate than frames whose information rate is equal to the full rate. This is due to the synergistic effects of combining interleaving with the repeating of the bits in the frame. The lower frame error rate of the frames having the lower information rate allows these frames to be transmitted at an even lower power. For example, for a frame whose information rate is xe2x85x9, the power can be reduced to below xe2x85x9 the power of a frame whose information rate is the full rate. The base station can adjust the power of a frame having an information rate lower than the full rate.
Referring to FIG. 1, in conventional CDMA systems, each forward traffic frame 10 (i.e., frames transmitted over the forward traffic channel) includes voice or data and error control information, typically in the form of a cyclical redundancy code (CRC). By contrast, each reverse traffic frame 20 (i.e., frames transmitted over the reverse traffic channel) includes voice or data and error indicator bits (EIB) for indicating whether the last forward traffic frame is a good frame or in erasure, i.e., a bad frame.
When base station 30 transmits forward traffic frame 10, mobile station 40 receiving forward traffic frame 10 will check the CRC to determine whether forward traffic frame 10 is good or not. Mobile station 40 will indicate such determination to base station 30 using the EIB in the next reverse traffic frame the mobile station will transmit. For example, a zero error indicator bit indicates no error in the forward traffic frame, and a positive error indicator bit indicates the forward traffic frame is a bad frame. Upon receiving reverse traffic frames from the mobile station, the base station examines the EIB and determines whether its forward link to the mobile station is in fading, and adjusts the power of its forward link accordingly. For example, if the base station receives one or more successive EIB, denoting erred forward traffic frames, the base station may determine that its forward link is in fading and increase the power of its forward link. This is to ensure that the frame error rate is kept to an acceptable percentage, typically between 1% and 3%, depending on the desired system performance.
Therefore, in a conventional CDMA wireless, communications system, a power control decision to either adjust the power or keep the power at its current level occurs once every frame, when the EIB is received. In newly proposed CDMA wireless communications system (hereinafter referred to as CDMA 2000), the forward link power control is much faster. The forward link power control is at 800 Hz rate, which means that power control information, referred to herein as a power control bit, is sent every 1.25 ms, or once for every power control group. Therefore, the base station cannot wait until the end of the forward traffic frame to determine if the power should be adjusted. Referring to FIG. 2, in CDMA 2000 power control is effected using slow outer loop 100 and fast inner loop 110. In outer loop 100 mobile station 120 determines a target signal to noise ratio using target frame error rate 124, which is typically between 1% and 3%, depending on the desired system performance. Signal to noise ratios are often expressed as the ratio Eb/N0, where Eb is the energy per information bit and N0 is the power spectral density of the interference seen by the receiver. Thus, target Eb/N0 130 can be used for the target signal to noise ratio. Target Eb/N0 130 is determined for each frame. Thus, for a 20 ms frame the speed of the outer loop is 50 Hz. After target Eb/N0 130 is determined, it is passed to inner loop 110. In inner loop 110, target Eb/N0 130 is compared to measured Eb/N0 160 of the received signal, which is measured for the 1.25 ms since the last comparison. When measured Eb/N0 160 is smaller than target Eb/N0 130 the mobile station 120 requests an increase in power. When measured Eb/N0 160 is larger than target Eb/N0 130 the mobile station 120 requests a decrease in power.
A problem with this system is that it does not allow frames whose information rate is lower than the channel rate to be transmitted at lower power than the full rate power scaled by the information rate scaling factor. For example, for a particular system, a frame having an information rate equal to the xe2x85x9 rate transmitted at {fraction (1/16)} the power of a frame at full rate may have an acceptable frame error rate, and because the information rate is xe2x85x9 the target Eb/N0 130 will be xe2x85x9 of a target Eb/N0 130 when the information rate is equal to the full rate. Base station 180 will transmit a frame having an information rate of xe2x85x9 at {fraction (1/16)} the power of a frame at a full rate, which will produce a measured Eb/N0 of about xc2xdthe target Eb/N0 130. Mobile station 120 receives the bits in the first 1.25 ms of the frame, and measures the Eb/N0 of these bits. Mobile station 120 then compares this measured Eb/N0 to the target Eb/N0 130. Because target Eb/N0 130 is larger than the measured Eb/N0, the mobile station will continue requesting an increase in power until the frame is being transmitted at xe2x85x9 the power of a frame at full rate, therefore eliminating the reduction in power that was available due to the synergistic effects of combining interleaving with the repeating of the bits in the frame.
The invention solves the above problems by providing a variable power control scaling factor to the mobile station. The mobile station determines a target signal quality measurement for transmitting a signal, and scales this target signal quality measurement by the variable power control scaling factor. Providing the variable power control scaling factor to the mobile station allows frames having an information rate lower than full rate to be transmitted at a power even lower than the information scaling factor times the power of the frame having an information rate equal to the full rate.
In another embodiment of the invention, the base station determines an information transmission rate of a signal to be transmitted. The base station then obtains a variable power control scaling factor based on the information transmission rate, and transmits the variable power control scaling factor.